Dirt Is Your Enemy

Dirt is the biggest problem in using fiber optics. When you are working with hair-thin strands of glass that must be mated in exact alignment, the tiniest specks of dirt can cause major problems. Dirt affects the loss of connections, the integrity of splices and the accuracy of tests during installation. When connecting communications equipment, the loss caused by dirt can be enough to prevent the link from working.

Where does the dirt come from? The air is the most obvious place, especially around construction sites. Typical airborne dust is about the size of the core of single-mode fiber and about 20 percent the size of the core of multimode fiber. Furthermore, airborne dirt may be hard silica, which can scratch a polished connector ferrule end when mated.

Fingers are another source of dirt and contamination, since your skin is oily. Touching the end of a connector ferrule may contaminate it with oil that will cause dirt to stick and would require aggressive cleaning. Then, improper cleaning may leave residue, making the connector dirtier than before!

Fibers exposed to the air may pick up dust quickly, so the installer needs to clean the fiber just before it is cleaved and placed in a splicing machine or inserted into a mechanical splice. Fusion splicers “prefuse” the fiber to warm it up and blast off any lingering particles of dust.

When terminating fiber, it should be cleaned just before inserting it in the ferrule for adhesive/polish fiber termination or just before cleaving for prepolished/splice connectors. After termination is completed, the finished connector should be protected with a dust cap.

Some installers say they always keep the dust caps on their connectors before mating them, so they should have no problems with dust or dirt. Dust caps provide excellent physical protection to the connector ferrule and should always be placed on the connector when it is unmated. But one installer I know claims they are called “dust caps” because they are always filled with dust. Connectors should always be cleaned after removing the dust cap and before mating. Dust caps also should be kept on mating adapters on patch panels to prevent dust buildup.

One of the biggest problems caused by dirt is testing error. Reference patch cables used in testing are mated repeatedly where they are susceptible to getting dirty from the air, mating adapters and the connectors they are testing. Connectors on test cables must be checked and cleaned often to prevent testing errors, especially before setting the “0 dB” reference.

One may determine if there is dirt on connectors by inspecting the connector ferrule with a microscope. The same microscope used for checking the polish on the connector during termination works just fine, but if one needs to inspect a large number of connectors at a patch panel, a portable video microscope will speed up the process.

How do you clean connectors? Well, don’t do what I saw a tech doing in one of my first visits to a fiber optic installation site 30 years ago. This tech blew on the connector and wiped it on his shirt before inserting it in a mating adapter on a patch panel! Ouch!

Proper cleaning uses professional cleaning kits or lint-free wipes and high grade isopropyl alcohol. One should always use lint-free wipes since cleaners such as cotton will leave a tangle of threads that can be worse than dirt. Do not use rubbing alcohol since it is high in water, and the moisture can be harmful to connections. Use only 99 percent--plus pure isopropyl alcohol, which will safely clean the connector without harming adhesives or plastics. After wiping with the alcohol, use a dry lint-free wipe to clean off the excess alcohol and dirt then inspect the surface again with a microscope. Be extremely careful with the alcohol because it is flammable.

Most of the new cleaning kits use special dry or sticky wipes for general cleaning and include a solvent for stubborn dirt. Often one swipe on a dry cleaning pad is all you need. Oily connectors may require wetting with the solvent and then cleaning and drying. Again, be careful with any solvents. Check the manufacturer’s safety instructions and always work in well-ventilated areas.

The final step in all cleaning processes is to inspect again with a microscope. Remember to keep the microscope clean, so the view is clear. If you are working on an operating link, be sure the link transmitter is turned off, especially if it is a high power laser or wavelength-division multiplexing system. To be safe, use a microscope with an infrared filter to block link power or a video microscope.

HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.